JPS61284554A - Alloy steel for unrefined bolt or the like having superior toughness and steel material for unrefined bolt or the like using same - Google Patents

Abstract

PURPOSE:To obtain an alloy steel for an unrefined bolt or the like having superior toughness by hot working a low-alloy steel having a specified composition and by cooling it under specified conditions. CONSTITUTION:An ingot of a low-alloy steel contg., by weight, 0.05-0.20% C, <0.05% Si, 1.0-2.0% Mn, <0.015% S, 0.01-0.05% Al and at least one among <0.15% V, <0.1% Nb and 0.05-0.50% Cr is heated to 950-1,150 deg.C and worked into a wire rod for a bolt by hot rolling. The wire rod is finish- rolled and cooled to <=500 deg.C at <=10 deg.C/sec cooling rate to form a dense ferrite- pearlite structure. A steel material having the desired strength and toughness and suitable for use as a material for an unrefined bolt or the like is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an alloy steel with excellent toughness for use in non-heat treated bolts, etc., and a steel material for non-heat treated bolts, etc. using the same.

[Background of the Invention] Conventionally, tempered bolts have been mainly used for bolts having a tensile strength of 70 kg/m2 or more, for example. Heat-treated bolts are made from medium carbon steel wire rods and bars with a carbon content of 0.3% by weight or more, or low alloy steel wire rods and bars, and are usually subjected to one spherical dead annealing treatment, cold wire drawing processing, It is manufactured through cold bolt forming, thread rolling, and finally quenching and tempering.

However, in recent years, with the increasing demand for energy saving, there has been a demand for so-called non-thermal bolts that can omit single-spherical dead annealing and heat treatment of quenching and tempering.
Therefore, alloy steel for non-tempered bolts is desired. In addition, there is a demand for steel materials for non-tempered bolts, which are intermediates for bolts and the like, which are made by rolling the alloy steel material and making wire rods, steel bars, and the like. In response to these demands, several steel materials for non-thermal bolts have been developed and put into practical use. One such method is to obtain a predetermined strength by precipitation strengthening based on low carbon steel. However, it has not yet become established.

The reason for this is that conventional non-tempered bolts made from this type of steel have lower toughness than tempered bolts.

Regarding the toughness of bolts, JIS 1051 stipulates mechanical properties for each strength category.1 Mechanical properties include nail tensile strength, head impact strength, etc. that evaluate the toughness of bolts. There is. However, when a part is manufactured by changing the manufacturing process, the reliability of the part is generally considered important, and this concept also applies to non-temperature bolts. Therefore, in addition to satisfying the JIS characteristics, tests are conducted under even stricter conditions. However, while tempered bolts not only satisfy the JIS standard values mentioned above, but can also achieve satisfactory values under even more hostile test conditions than JIS standards, For non-tempered bolts, for example, if a wedge tensile test, which is one of the physical tests for bolts, is performed under test conditions that are stricter than JIS, if the wedge angle exceeds lOo, a fracture under the neck will occur. The value is also low.As shown above, non-tempered bolts made from conventional non-tempered bolt steel materials did not have good toughness.

The present invention has been made as a result of intensive research to solve the above-mentioned problems.

[Summary of the Invention] The first invention according to the present application has a carbon content of 1% by weight and a C0.05 to 0.
20%, Si less than 0.05% 1Mn L, O~2
．． 0%, so, ois% or less, AI 0.01~
Contains 0.05% and V 0.15% or less, Nb
An alloy steel for non-tempered bolts etc. with excellent toughness, containing at least one selected from 0.1% or less and 0.05 to 0.50% Cr, with the balance consisting of iron and unavoidable impurities. be.

The second invention according to the present application is 1% by weight, co, os ~
0.20%, S r less than 0.05%, Mn 1.0
~2.0%, S 0.015% or less, Al 0.05%, and at least one selected from V 0.15% or less, Nb 0.1% or less, Cr 0.05 to 0.50%.
An alloy steel material containing seeds and the balance consisting of iron and unavoidable impurities is heated to 950 to 1150°C and then hot rolled,
This is a steel material for non-tempered bolts etc. with excellent toughness, which is cooled to 500°C or less at a cooling rate of 10°C/second or less after finish rolling.

C is 0.05-0.20%. C is necessary to impart the required strength to steel through solid solution strengthening, and for this purpose it is necessary to add at least 0.05%. But 0
．． If it exceeds 2%, the toughness will deteriorate and the workability when forming into bolts will also decrease, so 0.05 to 0.2
%.

Si is less than 0.05%. Because Si is usually deoxidized,
Alternatively, it is added for solid solution strengthening, but in the present invention it is added to ensure toughness. For this purpose less than 0.05% is required. However, if it exceeds 0.05%, toughness will be impaired.

Mn is 1. Q ~2.0%. Mn is an element that has deoxidizing and desulfurizing effects, and also improves hardenability.

Moreover, it lowers the transformation temperature and refines the structure. Therefore, it is useful for improving toughness, and 1.0% or more is required to have such an effect. However, 2.0%
The upper limit is set at 2.0% because the toughness deteriorates when the content exceeds 2.0%.

S is 0.015% or less. S is an element that tends to segregate, and in order to reduce the amount of MnS-based nonmetallic inclusions and improve toughness and workability, it is necessary to keep the content to 0.015% or less.

AI is 0.01-0.05%. AI is added for deoxidation, but if it is less than 0.01%, it will not have this effect.
Moreover, if it exceeds 0.05%, the cleanliness will deteriorate and the toughness will be impaired.

Furthermore, it contains at least one selected from V, Nb, and Cr. When contained, the content is V 0.1
5% or less, Nb 0.1% or less, Cr 0.05-0
．． It is 50%. These elements are included to impart strength.

Cr imparts strength by improving hardenability.

However, if it is less than 0.05%, it has no effect.

Furthermore, if the content exceeds 0.5%, the effect will be saturated, so the upper limit is set at 0.5% for economic reasons.

V and Nb impart strength through grain refinement and precipitation effects due to the precipitation of fine carbides and nitrides during rolling and during the cooling process of the steel after rolling.

In addition, when V and Nb are contained, the toughness and workability are also improved due to the above-mentioned crystal grain refinement and precipitation effect, and also because of the grain refinement of the hot-rolled billet. However, if the content exceeds 0.15% in the case of ■ and 0.1% in the case of Nb, the effect is saturated, and for economic reasons, the content should be lower than that.

Next, in the second invention according to the present application, 950 to 115
Hot rolling is carried out after heating to 0°C. After the precipitates are dissolved in the matrix, fine carbides and nitrides are precipitated to refine the crystal grains and harden the crystals, or to improve the initial crystallization. A temperature of 1150°C is sufficient for grain refinement;
If the temperature is less than 50°C, deformation resistance increases during hot rolling and productivity decreases, so the lower limit is set at 950°C.

In the present invention, after finish rolling, 10
Cool at a cooling rate of ℃/second or less. This makes the structure a dense ferrite/pearlite structure and provides the desired strength and toughness. If the cooling rate exceeds 10° C. for 1 second, bainite will be present, the strength will be higher than desired, and it will be difficult to obtain toughness. Furthermore, it also reduces the workability when forming into bolts.

The alloy steel according to the first invention and the steel material according to the second invention can be used, for example, to create bolts, pins, and rebates.

Incidentally, the steel used in the present invention is melted, for example, in an LD converter, but the melting method is not limited at all.

[Embodiments of the invention] Examples will be described below with reference to comparative examples.

Steel having the chemical composition shown in Table 1 is melted in an LD blast furnace,
The steel material (rolled material) as a wire rod was obtained by rolling into a 6-inch diameter wire rod under the rolling conditions shown in Table 2. In Tables 1 and 2, number 12 is a conventional steel material for tempered bolts.

A tensile test was conducted on these rolled materials. The results are shown in Table 2.

Next, the rolled material was drawn to a thickness of 5.22 mm to produce a steel wire. When this steel wire was subjected to a tensile test, good results were obtained in the example, especially in terms of drawing (Table 2)6
Next, this steel wire was formed into a hexagonal upset bolt with a thread size of M6, then subjected to bluing treatment, and for No. 12, which created a non-tempered bolt, a spherical dead annealing treatment was performed, followed by slight wire drawing. No cracking occurred during bolt creation, except for No. 0 No. 10 (comparative example), which was quenched and tempered after bolt forming and thread rolling.

A physical bolt test was conducted on the bolt produced as described above. According to JIS, the R under the neck of a hexagonal bolt with thread size M6 (R in Figure 1) is a minimum of 0.2.
Although there are 5 layers, in this test, R was 0.1 and the fat layer was set as 0.1.

The tests were conducted using a wedge tension test and a head impact test.

Wedge angle in wedge tension test (α in Figure 2)
) is a maximum of 10° according to JIS, but in this example it is 10°.
It was set to 5'.

Furthermore, the angle in the head impact test (β in Figure 3)
) is BO~80° according to JIS.

In the example, the angle was set to 50°.

In other words, the test conditions were made extremely strict compared to JIS.

Table 2 shows the above test results.

As is clear from Table 2, it can be seen that the bolts made from the steel materials according to this example have properties comparable in toughness to those of heat-treated bolts, even if they are non-tempered bolts.

In addition to those listed in Table 4, we also conducted bolt fatigue tests, relaxation tests, and guaranteed load tests, and it was confirmed that even non-tempered bolts had properties comparable to tempered bolts. are doing.

[Effects of invention According to the present invention, the following effects can be obtained.

By using the alloy steel and steel material according to the present invention, non-tempered bolts and the like with excellent toughness can be obtained. In particular, it is possible to obtain non-tempered bolts, etc., which can satisfy the standard values higher than those stipulated in JIS, and have characteristics comparable to those of heat-treated bolts, etc.

[Brief explanation of drawings]

FIG. 1 is a side view showing the bottom R of the bolt neck. FIG. 2 is a cross-sectional view showing the wedge angle in the wedge tensile test. FIG. 3 is a cross-sectional view showing angles in a head impact test. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1% by weight, C 0.05-0.20%, Si 0.
Less than 0.05%, Mn 1.0-2.0%, S 0.015
% or less, containing 0.01 to 0.05% Al, and
V 0.15% or less, Nb 0.1% or less, Cr 0.
1. An alloy steel for use in non-thermal bolts, etc., which has excellent toughness and is characterized by containing at least one element selected from 0.05 to 0.50%, and the balance being iron and unavoidable impurities. 2% by weight, C 0.05-0.20%, Si 0.
Less than 0.05%, Mn 1.0-2.0%, S 0.015
% or less, containing 0.01 to 0.05% Al, and
V 0.15% or less, Nb 0.1% or less, Cr 0.
95% alloy steel material containing at least one selected from 0.05 to 0.50%, with the balance consisting of iron and unavoidable impurities.
A steel material for use in non-tempered bolts etc. with excellent toughness, characterized by hot rolling after heating to 0 to 1150°C, and cooling to 500°C or less at a cooling rate of 10°C/second or less.